This invention relates generally to information storage devices and, more particularly, to thin film magnetic domain devices.
1. Field of the Invention
A single wall or bubble domain for the present invention is defined as a magnetic domain bounded by a domain wall which closes on itself in the plane of a host magnetic medium and has a geometry independent of the boundaries of a sheet of the medium in the plane in which it is moved. The term bubble domain includes circular wall-shaped domains, elongated circular or stripe domains, and segment domains where a portion of the domain boundary is completed by a magnetic discontinuity such as a boundary of the sheet. Inasmuch as a bubble domain is self-defined in a plane of movement, it is free to move in two dimensions and such a plane is now well known. The movement of domains is normally performed by generating a localized field gradient within the host magnetic medium in the desired direction.
Materials which are well known in the art for their ability to support bubble domains are rare earth orthoferrites and garnets. These materials have preferred directions of magnetization substantially normal to the plane of the sheet. A bubble domain, in a material of this type, is magnetized in one direction along its axis whereas the remainder of a sheet is magnetized in the opposite direction, the domain appearing as a dipole oriented normal to the plane of the sheet. Other magnetic materials may be used as bubble domain carriers so long as the magnetic material is anisotropic with the easy axis of magnetization normal to the plane of the sheet.
A confined array such as the lattice array of bubble domain elements can be formed by confining a plurality of bubble domains while providing accessing means to enter and remove bubble domains into and out of the confined array. The bubble domains themselves store the necessary data information. Since each bubble domain itself stores the required data, the bubble domain can be placed close together and confined. The interaction forces between domains stabilize their position within the lattice array. The lattice array is, therefore, an efficient information storage device. Reference is made to a co-pending U.S. patent application, Ser. No. 395,336, filed on Sept. 7, 1973 now abandoned and assigned to the assignee of the present invention, for a complete description of a lattice array of interactive elements.
2. Description of the Prior Art
In most of the prior art information storage systems using single-wall magnetic domains, data bits were generally represented as the presence and absence of bubble domains. This method was used initially in the development of magnetic domain systems since the nucleation or splitting of bubble domains or not at will was perhaps the easiest method of adapting the technology to a practical use.
Different domain properties useful for storage of information and the retrieval thereof were then discovered. In a co-pending U.S. Pat. No. 3,911,411, filed on Dec. 29, 1972 and assigned to the same assignee as the present invention, domains having different apparent sizes are used to represent different information states. Another bubble domain property useful for storing information is that the magnetization in the wall of the domain can have different directions of rotation or chirality. The different directions of the rotation of the domain wall magnetization can be used as a binary representation of data. A publication by George Henry in the IBM Technical Disclosure Bulletin, Vol. 13, No. 10, p. 3021, March 1971, discloses the adaptation of chirality to the bubble domain apparatus.
In the work being performed on magnetic bubble domains, it was discovered that magnetic domains have other dissimilar properties which serve to distinguish one from another. It was discovered that domains can have different numbers of vertical Bloch lines which may roughly be thought of as a vertical line of twist in the wall magnetization. These Bloch lines separate two areas within the domain wall magnetization. It was further found that these different domains have different directions of movement in an applied gradient magnetic field.
The phenomena of the Bloch line was noted in a co-pending U.S. Pat. No. 3,940,750, filed on Mar. 26, 1973, and assigned to the assignee of the present invention. Since the discovery of the Bloch lines, there have been many articles discussing their properties including an article by A. P. Malozemoff, in Applied Physics Letters 21, p. 149 (1972) where it was shown that if there are enough vertical Bloch lines inside the domain wall, then the domain will collapse at a higher bias field than one with a smaller number of vertical Bloch lines. In addition, the diameter and mobility may be different depending on the number of vertical Bloch lines.
A utilization of these different types of magnetic bubble domains was included in a co-pending U.S. Pat. No. 3,890,605, filed on June 29, 1973, and assigned to the assignee of the present invention. The important property of different angles of deflection of the bubble domains in a gradient magnetic field according to the wall topology or state of the domain was recognized and disclosed therein. The angle of deflection was disclosed as a function of the number of rotations of wall magnetization around a peripheral of the domain wall and was used to represent data in an information store. However, the generation of the bubble domains was essentially haphazard in that a multiplicity of domains were generated without regard for their properties. These domains were first sensed according to their deflection angle and then stored in a data bit bin according to the deflection angle sensed. A particular bit bin was then actuated to release a domain according to the data state required for storage into the information storage system. Only the propagation deflection trait of the different domains were known at that time. One domain when propagated in a common field gradient could deflect at a different angle from another domain. However, the controlled generation of domains having selected wall states was not known.
Accordingly, it is an object of the present invention to provide a magnetic domain arrangement having controlled generation of wall states of bubble domains.
It is a further object of this invention to provide a method for the controlled generation of wall-encoded bubble domains.
Another object of this invention is to provide a method and apparatus to predictably switch the wall state of a bubble domain from an unknown state to a known state.
Yet another object is to provide an information storage device which utilizes two states of the magnetic bubble domain wall for binary data storage.
The use of a second magnetic layer on a bubble medium and its effect on the apparent elimination of "hard" bubble domains is given in an article entitled The Effect of a Second Magnetic Layer on Hard Bubble, by A. Rosencwaig, appearing in the Bell System Technical Journal, Vol. 51, pp. 1440-1444, July-August 1972. The purpose of the article was to eliminate the problem which developed because all bubble domains did not propagate in an orderly fashion. The second or capping layer without a unipolar in-plane field, according to this article, reduced the number of Bloch lines to one pair. This was found to be incorrect by my experiments.
It is therefore another object to provide a method and apparatus for controllably generating bubble domains of two known wall states in a bubble medium having a second magnetic layer.
These and other objects of the present invention will become apparent to those skilled in the art as the description proceeds.